Where Planets End and Small Stars Begin Depends on the Star's Mass

A survey of 63,254 stars finds that brown dwarfs, the objects between planets and stars, go missing as close companions at a fixed fraction of the host star's mass, close to 2%, rather than at a fixed mass. William Thompson of NRC Herzberg Astronomy and Astrophysics in Victoria, British Columbia, and colleagues report that companions are strongly depleted at that ratio from small M dwarfs up to hot B stars. The work is a preprint, posted Nov. 9, 2026, and has not been peer reviewed.

The authors argue that this makes the ratio of the two masses, not the companion's mass, the meaningful line between planets and star-like companions, that it shifts with the star, and that the gap marks the divide between two ways a companion can form: building up inside a disk below it, and collapsing out of a fragmenting cloud above it.

In the preprint, the team fit a single model to astrometry, the measurement of a star's position, from the Hipparcos satellite and Gaia's second and third data releases, covering companions on orbits of 3 to 94 years. Fitting the gap separately in each host-mass range put its floor at 1.76% of the host's mass across K through B stars. In the fit to the whole sample, companions at the floor turn up tens of times less often than at the peaks on either side.

The two populations overlap in mass, so one mass cut cannot describe both: the lightest star-like companions of K stars come in near 13 times Jupiter's mass, below the heaviest planet-like companions of B stars, which reach roughly twice that. Some companions below the gap are heavier than the deuterium-burning limit, the usual dividing line between a planet and a brown dwarf, and still look like a smooth continuation of the planet population, the authors write.

Companions below the gap are more common around stars rich in heavy elements, and those above it rarer, which the authors take as support for two formation routes. The paper has been submitted to The Astrophysical Journal Letters.